The answer is clear as a day and it is cryptocurrency trading.
The forex market has a massive daily trading volume exceeding $5 trillion. It is the most liquid market on earth. Crypto trading is nowhere close to this and have a daily trading volume in the billions. At the time of writing this post, the trading volume of crypto in the last 24 hours was a little higher than 11 billion (see https://coinmarketcap.com).
The crypto market is far more volatile than forex due to the massive difference in liquidity. Even among cryptos, the ones that have more liquidity such as Bitcoin, Ethereum etc have lower volatility than low liquidity coins like Req Token or Steem.
That is why coin or state currencies with high liquidity have lower risk. So, crypto trading is on average, much more risky than forex.
In terms of volatility, it may seem like Cryptocurrency trading is much riskier than forex. However, there is one key factor in trading, leverage. In cryptocurrency trading, most of the trades are not leveraged. Which means that it is not possible to lose more than what you have. However, in Forex, most trades are leverage (up to 200x). On that basis, I am more inclined to say that forex trading is riskier because the potential downside is so much higher.
That being said, with the recent establishment of leveraged crypto trading platform, such as Bitmex, we should see more risky leveraged crypto trading moving forward. Leveraged crypto trading will probably be the riskiest in the world.
Depending on how you TRADE these instruments. Trading has risks on it's own. First of all, if you treat crypto as crypto, currency as currency by their nature of usage, you'll have no problem at all. What makes FOREX a lucrative trading business, simply because broker offer huge leverage to their client, whereas crypto trading virtually none existed with margin trading. To me, they're equally risky. When one over leveraged their trading capital in FOREX, a minor move in the market can cause margin call. When one dump money they can't afford to lose in crypto, they could get stuck in crypto financial crisis.
As for myself, I treat crypto as a separate entity. Whatever fiat currency I had invested into crypto, I treat them as burnt. I use crypto as my commodities. I trade between cryptos to earn cryptos. When I have enough crypto income, I liquidate them into fiat and I call that profit. If I don't liquidate them, those crypto will be in my wallet or invest into development. In Steem case, load them as Steem Power and upvote people and or delegate to bot as investment.
Without a doubt, there is MORE risk in cryptocurrency trading.
I have been involved in Forex trading before and the only way it gets risky is when you use leverage option (high risk and liquidity concerns). Your account get a margin call in those scenarios.
Swings of 10% or more is not common in Forex on a day-to-day basis. But no one bats an eye when it happens to Bitcoin or Altcoins. Actually, people get skeptical when the market moves sideways or relatively flat in the cryptocurrency market.
The good news is that the Forex market is in trillions of US Dollars while the crypto market is just growing! There is a long way to go.
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The short answer, cryptocurrency trading.
This is because despite both trading involves heavy speculations, the cryptocurrency market has far less liquidity as compared to forex market. Also, cryptocurrency is actually backed by some invisible underlying technology while fiat currencies of respective countries are actually backed by the federal reserve of the said country. In another words, basically cryptocurrency trading is way more dangerous compared forex trading due to the security concerns.
As for now, the early budding stage of cryptocurrencies are filled with tonnes lot of ICOs which are actually unregulated. There are various cases of scam happening in the cryptocurrencies market due to this exact reason. That's why it is definitely more risky.